In a drilling rig having a full complement of drill pipe and tool joints connected in a drill string, the overall weight of the string is large compared to the desired weight at the drill or coring bit. In coring operations especially, the desired bit weight (WOB) can be as little as 1/2 of one percent of the total drill string weight. Using existing conventional weight indicators, the accuracy of sensing and controlling is prohibitive for maintaining the WOB within such a narrow range.
During the drilling of boreholes using a rotary bit, it is desirable to maintain a constant WOB because wide variations of the weight on the bit tend to wear out or damage the bit prematurely and therefore reduce the rate of penetration. Coring especially requires an accurate, constant sensing of the drill string weight to make possible a smoother braking of the block travel while drilling. The automatic drilling system of this invention controls the bit penetration rate within the operating range of 20 meters/hour to 0.7 meters /hour while maintaining a set bit weight. Given a definite bit weight set point between 0 and 15,000 Kg, the bit weight controller can maintain the bit weight within 200 Kgs. This is an unusual and unexpected result when it is considered that the drill string weight often exceeds 100,000 kg.